1. Field the Invention
The present invention relates to electrical connectors for connecting to a mother board a daughter board having a plurality of pads on a front edge thereof in a direction perpendicular, oblique, or parallel to the mother board.
2. Description of the Related Art
Recently, an increasing number of memory modules are used and there is a demand for an electrical connector for allowing high-density mounting. A conventional direct insertion type connector fails to meet the high-density mounting requirement and a variety of rotary type, zero-insertion-force connectors are used.
Japanese patent application Kokai Nos. 230378/85 and 193473/88 disclose such zero-insertion-force connectors. Contact terminals for the electrical connectors are made by stamping a metal sheet having a predetermined thickness. These contact terminals have a very high spring constant because they are stamped from flat work. Consequently, if the spring constant is set to provide a satisfactory contact power for PC boards of the minimum thickness, an excessive force is applied to PC boards of the maximum thickness, causing breakage or difficulty to plug. In addition, the contact terminals are made by stamping so that a considerable amount of rare metal material is wasted. Moreover, the contact terminals made by stamping have contact sections cut by the stamping so that the contact sections must be plated after stamping, resulting in the increased manufacturing costs.
Japanese patent application Kokai No. 78168/90 discloses contact terminals made by stamping and bending a metal sheet instead of those made by stamping alone. By stamping and bending it is possible to avoid the above problems with those made by stamping alone.
However, such an electrical connector has the following disadvantages. That is, since a daughter board is inserted and then rotated, the moment of rotations (in a direction to release the daughter board) warps the daughter board, or the positions of contact points of contact terminals vary with variations in the manufacturing precision, making uneven the contact power of the contact terminals. The warp of a daughter board makes different the contact power in upper and lower contact points and unstable the contact resistance.
Such disadvantages will be described in more detail with reference to FIGS. 13 and 14. As FIG. 13 shows, an electrical connector of this type is mounted on a mother board 10 to connect a daughter board 30 such as a printed circuit board on which memory modules are mounted. The electrical connector 20 has an insulation housing 21 with an elongated opening 25 therein and a pair of latch levers 22 extending upwardly from opposite ends of the elongated opening 25 and having latch sections 23 at the upper portions thereof. A plurality of contact terminals are arranged in the insulation housing 21 along the elongated opening 25. To connect the daughter board 30 to the electrical connector 20 on the mother board 10, the daughter board 20 is inserted obliquely into the elongated opening 25 and rotated rearwardly. When the side edges of the daughter board contact the front faces of the latch sections 23, the daughter board 30 flexes the latch levers 22 outwardly and passes the latch sections 23. When the daughter board passes the latch sections 23, the latch levers snap to the original position to hold the daughter board 30 with the rear faces of the latch sections 23. This completes connection of the daughter board 30 to the electrical connector 20. FIG. 13 shows such connection conditions of the daughter board 30 to the electrical connector 20.
Under such connection conditions as shown in FIG. 13, the daughter board 30 is biased to rotate forwardly by the contact terminals while the upper opposite side edges of the daughter board 30 are held by the latch sections 23 of the latch levers 22 to prevent the forward rotation. Consequently, the higher the moment of forward rotations applied to the daughter board 30 by the contact terminals, the larger the warp of the daughter board 30 as shown with an arrow W in FIG. 13. As FIG. 14 shows, the contact power of the rear contact terminals provided in the middle of the elongated opening 25 is decreased while the contact power of the front contact terminal provided at opposite ends of the elongated opening 25 is decreased, failing to provide stable two-point contacts.
The propositions made to solve such problems include reduction of the contact power of the contact terminals to thereby reduce the moment of rotations, minimizing the warp of a daughter board; holding projections molded with the insulation housing to correct the warped daughter board; and contact terminals made by drawing as shown in Japanese UM patent application Kokoku No. 9347/95.
However, the reduction of the contact power increases the contact resistance, reducing the contact reliability; the molded holding projections fails to meet the tolerance in thickness of daughter boards; and the drawn contact terminals have their own disadvantages.